1. Field of the Invention
The present invention relates to a multi-layer structure for a II-VI compound semiconductor thin film and a method of forming the same.
2. Description of Related Art
Semiconductor lasers for wavelengths from an infrared region to a red region and light emitting diodes for wavelengths from the infrared region to an olive region are realized using a III-V compound semiconductor composed of the group III element such as Al, Ga, and In and the group V element such as As, P and Sb. In order to realize light emitting devices with a shorter wavelength than such wavelengths, a semiconductor having a wider energy band gap is required. However, it is difficult to achieve the wider energy band gap in the III-V compound semiconductor as mentioned above.
On the other hand, II-VI compound semiconductor composed of a group II element such as Zn, Cd, and Te and a group VI element such as S, Se and Te have a relatively wide energy band gap to emit light over the visible spectral region. For this reason, the II-VI group compound semiconductors are expected as the material for light emitting devices from green to ultraviolet spectral region and have been investigated intensively.
Since it is difficult to obtain II-VI bulk crystals with good quality, a III-V bulk crystal which is commercially available with a high quality is usually used as a substrate to fabricate a II-VI compound semiconductor film. The lattice constant of GaAs is close to that of ZnSe which is one of basic alloy in II-VI compound semiconductor so that GaAs is most generally used in III-V bulk crystal as a substrate to grow the II-VI compound semiconductor thin film. However, by using InP as the substrate material, II-VI compounds which are a different composition from that of the II-VI film grown on the GaAs substrate under the lattice-matching condition can be grown and is used in many application fields.
FIG. 1 is a layer structure of a conventional II-VI compound semiconductor film manufactured on an InP substrate by a molecule beam epitaxy (MBE) method. In a case where the II-VI compound semiconductor is manufactured on the InP substrate 1, it is necessary to remove a native oxide layer on the surface of the InP substrate 1. Usually, in a case where a III-V compound semiconductor thin film is formed on the InP substrate 1, the removal of the native oxide layer is performed while irradiating a beam of group V atoms such as P or As, so that desorption of P from the InP substrate 1 is prevented. However, in case of II-VI compound growth, because group V element such as P and As acts as impurity. For this reason, in order to remove the native oxide layer from the substrate while irradiating the beam of group V atoms, it is necessary to provide a III-V compound semiconductor growth chamber connected to a II-VI compound semiconductor growth chamber via a vacuum carrying mechanism or a new dedicated chamber for irradiating a beam of group V atoms. As a result, the MBE apparatus becomes very complicated and expensive so that the manufacturing cost of a device is increased.
In case of an MBE apparatus with no III-V element compound semiconductor growth chamber or no chamber for irradiating the beam of group V atoms, the InP substrate 1 is heated up, without irradiating the beam of V group atoms, to 480.degree. C. at which the native oxide layer on the InP substrate surface is decomposed and vapored. Then, after it is ensured by means of the in-situ monitoring such as a reflection high energy electron diffraction that the native oxide layer is removed and 2.times.4 surface reconstruction appears, the substrate temperature is immediately decreased to minimize the desorption of P atoms. Many In metal droplets are observed on the InP substrate surface from which the native oxide layer is removed by the above method. This is because P atoms in InP substrate violently desorbs from the substrate surface in addition to the decomposition and vaporing of the native oxide layer if the InP substrate 1 is heated to 480.degree. C. without irradiating the beam of group V atoms. If a ZnCdSe thin film which is one of II-VI compound semiconductor films is formed on the InP substrate having droplets, the ZnCdSe cannot be correctly grown on the In droplet portions. FIG. 2 is a diagram showing a surface morphology of the ZnCdSe thin film having 0.6 .mu.m thickness which is formed on the InP substrate 1 in this manner. The photograph was taken with a Normarski phase contrast microscope. White portions indicate defective portions where abnormal growth is caused due to In droplets. Pits are formed at the white portions. The thin film having the many pits cannot be applied to a device.